Yes, thank you for not arguing from the bible, since that presupposes we agree on its relevance and authenticity.

The planet is over crowded, so the child production only matters to those still having children. If we follow the same curve as many other countries (like in the EU), I don't foresee that our baby making rates will continue as they have.

However, I don't disagree that it's good to have both parents ready willing and able to parent - but what does that have to do with marriage?

Marriage is a governmental and/or religious institution. A committed relationship can exist outside of legal documents. And signed documents do not a lasting marriage make.

Legally binding yourself to another, with the possible exception of protections for the children, is crazy.